Known vehicle powertrain systems include internal combustion engines and electric motor/generators that may be coupled to transmissions to transfer torque to a driveline of a vehicle for tractive effort. Known electric motor/generators are supplied electric power from high-voltage energy storage systems. Powertrain systems may employ regenerative control systems to recover electric power for charging the high-voltage energy storage system in response to operator commands that include braking and/or coasting. In one embodiment, torque may be transferred between the engine and the electric motor/generator via a belt-driven mechanism that rotatably couples the engine crankshaft and the rotor of the electric motor/generator. Mechanical vibration may be generated during torque transfer between the engine and the electric motor/generator due to various design and operating characteristics. The mechanical vibration may include periodic ripples that may generate an acoustic noise.